Rain cloud prompts infirm regulars to take vehicle (6)
I believe the answer is:
nimbus
'rain cloud' is the definition.
(I know that rain cloud can be written as nimbus)
'infirm regulars to take vehicle' is the wordplay.
'regulars' indicates alternate letters (letters selected at regular intervals).
'to take' says to put letters next to each other.
'vehicle' becomes 'bus' (public transport vehicle).
The alternating letters of 'infirm' are 'nim'.
'nim'+'bus'='NIMBUS'
'prompts' acts as a link.
(Other definitions for nimbus that I've seen before include "A black rain cloud" , "Ouse/Trent estuary" , "Dirty cloud; saintly light" , "it could bring rain" , "Is numb (anag)" .)